1. Field of the Invention
The present invention relates, in general, to an organic optical data storage medium which is recordable and readable by using a light source of laser and, more particularly, to an organic optical recording medium with high data storage density, high data rates and long data archival capabilities, useful as a medium for recording a variety of information or pictures.
2. Description of the Prior Art
A variety of information is explosively increased in an information-intensive society. Such information explosion requires recording media to have increased data storage density and data rate and to be more rapid in operation.
Currently, the practical or commercial techniques for recording data are based substantially on magnetic recording technology. In general, the data are stored on magnetic media, such as video tapes, audio tapes, floppy disks and the like, on which information is recorded depending on the direction of magnetization of magnetic substances in the magnetic recording medium.
While the magnetic recording technology is commercially successful and advantageous, a recording technique known generically as optical recordings has been and continues to be considered a very promising alternative for data storage, as a recording medium with higher capacity is demanded according to the enormous amount of information resulting from the development of society.
A magneto-optical recording medium comprises a recording layer magnetizable in a plane perpendicular to the plane of the layer itself, in contrast with magnetic media. In addition, the coercive force of a magneto-optical recording medium, which is the ability to remain in the magnetized state, is about 5 to 10 times as high as that of a magnetic medium. Accordingly, it is very difficult to change the previous direction of magnetization with an external magnetic field.
The recording of information on a magneto-optical recording layer is effected by first focusing a modulated laser beam on a surface of the layer within 1 .mu.m in diameter, the laser beam power being sufficient to heat the layer locally, for example, to the Curie temperature point of the layer. In this state, the direction of magnetization can be changed with an external magnetic field, so as to record information on the layer according to the direction of magnetization.
When the information is recorded by this method, the recorded unit of information comes to be reduced into 1 .mu.m or less. Accordingly, the recording density of magneto-optical recording medium is 10 to 1,000 times greater than that of conventional magnetic recording medium. In addition, the magneto-optical medium employs a non-contact recording method, so that magneto-optical recording potentially has significant advantages over magnetic recording, including easier data preservation and longer data archival capability.
However, there are many disadvantages in producing the magneto-optical recording medium used in such method. For example, heavy metals are used as magnetic substances and a very expensive vacuum deposition or sputtering apparatus is required.
Many attempts have been made to solve such disadvantages. One of the attempts is to develop an organic optical recording material. The organic optical recording material may be grouped into a Write Once Read Many (hereinafter, referred to as "WORM") type and a rewritable type on the basis of the erasability of the material. On the WORM type material, only readout of information is possible after recording data once, whereas, on the rewritable type material, erasure of the data is also possible after recording.
WORM type medium is manufactured, as disclosed in Japanese Patent Laid-Open Publication Nos. Sho. 57-46362, 58-197088, 59-5096 and 63-179792, by coating light-absorbing dye admixed with polymer on a reflective layer to form a recording layer and overcoating a protective layer on the recording layer. To write a data bit in this optical recording system, a laser beam is focused on a very small spot of the recording layer, for example, within 1 .mu.m in diameter. As a result, the light-absorbing dye generates heat, which decomposes the polymer to form a pit. Reading of the recorded information (logic 1 or 0) is effected by using the difference between the reflectivity of the medium in the presence and absence of the pit. Since WORM type material has a recorded portion which is in a state of polymer decomposition, it is impossible to record data on the polymer-decomposed portion after erasing the information.
Rewritable type material has been vigorously researched and the research has been directed to formation and erasure of bumps or pits or to use of liquid crystals or phase changes (Japanese Patent Laid-Open Publication Nos. Sho. 58-199345, 63-74135, 3-256241, 3-256242 and 3-266235). Of these, a method of utilizing bumps is extensively applied. In the method of utilizing bumps, recording is accomplished by thermally expanding a dye-dispersed organic polymer by a recording laser, to produce bumps and maintaining their shapes.
Technologies for such rewritable type materials employing the organic dye are described in many patents including U.S. Pat. Nos. 4,712,625, 4,780,867, 4,825,430 and 4,896,314, all being assigned to Optical Data Inc., USA. The rewritable type optical recording medium consists typically of three layers: a substrate, a recording layer and an erasing layer. While the recording layer comprises an elastic polymer resin and dye capable of absorbing a recording laser, the erasing layer comprises a thermoplastic polymer resin and dye capable of absorbing an erasing laser.
Reading of recorded information in the rewritable type optical recording medium is effected by using the difference between the reflectivity of unrecorded portions and bumps upon irradiating with a recording laser with a weak to medium power. However, the rewritable type optical recording medium possesses the characteristics of low reflectivity. To improve the reflectivity of the dye itself dispersed in the recording layer, there have been many attempts (Japanese Patent Laid-Open Publication Nos. Sho. 58-112790, 62-146682 and Hei. 1-206093). It has been proven that there is, however, a limitation in improving the reflectivity of the dye itself.
For erasure of data in the rewritable type optical recording medium, an erasing laser is initially focused on the erasing layer with a power sufficient to heat the erasing layer to not less than its glass transition temperature. As a result, the erasing layer is weakened in resistance to retention force of the recording layer, so that the recorded portion is restored to its original state by the retention force. In this regard, reliability should be taken into account for bump formation, in order to show appropriate information carrier to noise (C/N) ratios when using the medium repetitively. An improvement in the reliability was made in Japanese Patent Laid-Open Publication No. Sho. 63-207691, but it is believed that the patent has limited practical application to optical recording medium.
Currently, WORM type optical recording media are of disk shape in practice and have been intensively advanced in the fields including data storage disk, recordable CD-Audio and Photo CD. Related techniques are disclosed in Japanese Patent Laid-Open Publication Nos. Hei. 2-67183, 4-76836 and 4-102242, all assigned to Fuji Photo Film Co. Ltd., Japan; Japanese Patent Laid-Open Publication Nos. Hei. 2-24437, 2-273339, 2-312020, 3-66042, 3-203694, 3-224792, 4-25493 and 4-28588, all assigned to TDK Corp., Japan; Japanese Patent Laid-Open Publication Nos. Hei. 3-203690, 3-203691, 3-203692 and 3-203693, all assigned to Pioneer Electric Corp., Japan; Japanese Patent Laid-Open Publication Nos. Hei. 2-132649, 2-132654, 21-32656, 2-132657, 2-206093 and 2-84384 and U.S. Pat. No. 5,090,009, all assigned to Taiyo Yuden Corp., Japan; and Japanese Patent Laid-Open Publication Nos. Hei. 3-46135 and 4-70380, all assigned to Sony Corp.
CD-ROMs described in the just mentioned patents are recordable and characterized by readability in conventional CDP or CD-ROM drives. Their recording layer consists only of dye, which does not improve reflectivity of the disk and causes increases in production costs. A WORM type organic optical recording medium having a recording layer consisting of a dye-dispersed thermoplastic resin is also mentioned in the above-noted patents. Nowhere is there described CD compatibility of the WORM type organic optical recording medium, whereas there are examples of applying it to a disk. If it is applied for CD-compatible disks, there might be difficulty in selecting solvents for coating or in controlling the thickness of the recording layer. Nevertheless, effects such as low cost and improvement in reflectivity are expected to be brought about due to reduction in the amount of dye.
Commercially available, existing CD-ROMs, which have a recording layer consisting only of a dye layer, are recordable and readable in a drive for CDs but have a characteristic that data recorded once are not erasable because it is of WORM type, which is impossible to erase. To provide erasability, CD-compatible inorganic type disks have been researched but are very expensive. In addition, there is a risk of data loss by oxidation. Owing to these reasons, such CDs are not generally available.
In the case that commercially available CD-ROMs, on which data are recorded by a CD-ROM exclusive recorder and which have unrecorded portions after recording, are loaded on the recording apparatus, undesired data may be appended in the unrecorded and thus recordable portions by, for example, operator error or intentional operation. In spite of the security problems, such as data manipulation, annexation and so on, there are no readily available protective means that prevent rewrite of data. In addition, where the CD-ROM is repetitively read many times, there is a risk that data might be damaged by the recording laser.
In general, recording media using organic optical recording substances have an unavoidable, fatal drawback in durability. This drawback has been considerably minimized by virtue of technique development. However, in case of CD-ROMs using dye alone, the dye is spontaneously discolored, which results in the loss of data.